The present invention relates to a radiometer for measurement of thermal radiation or brightness temperatures, and more particularly to calibration of a radiometer with reference temperatures from an adjustable noise source providing warm radiation temperature signals and cold radiation temperature signals adjustable in accordance with an input to the adjustable noise source.
Radiometers are used to measure thermal radiation or brightness temperatures emitted from a segment of a remote object. The segment is commonly referred to as a scene and may be a portion of the earth""s surface. Like most sophisticated instrumentation, radiometers require periodic calibration to insure accurate measurements.
Numerous types of microwave/millimeter wave (MMW) radiometers have been proposed over the years. However, most prior art radiometers are subject to internal temperature gradient errors tending to obscure temperature determinations. In addition, electrical noise introduced through the antenna path and within the internal circuitry of the radiometer tends to obscure and adversely affect accurate measurements. Further, amplifier gain instability often causes distortion of the measurement signals. To overcome the latter problem, it has become common to employ a Dicke radiometer circuit wherein the signals from the antenna are sampled and compared with signals from a reference source maintained at a known constant temperature. This overcomes some of the problems of amplifier instability, but in general does not alter effects due to imperfect components and thermal gradients.
While other types of radiometric devices have been used with some success, the Dicke or comparison type of radiometer has been the most widely used for the study of relatively low level noise-like MMW signals, especially where the noise signals to be examined are often small in comparison to the internally generated noise level within the radiometer receiver. While there are several types of comparison radiometers, one popular type of radiometer for use in the microwave/millimeter wave frequency bands is that in which an incoming signal to be measured and a standard or calibrated reference noise signal are compared. This type of radiometer consists essentially of the comparison of the amplitude of an unknown noise signal coming from the source to be examined with a known amplitude of a noise signal from a calibration source. This method has been found useful in measuring with considerable accuracy the effective temperature of an unknown source. In the Dicke or comparison type radiometer, the receiver input is switched between the antenna and a local reference signal noise generator. The detected and amplified receiver output is coupled to a phase-sensing detector operated in synchronism with the input switching. The output signal from such a radiometer receiver is proportionate to the difference between the temperature of the reference signal source and the temperature of the source viewed by the antenna inasmuch as the phase-sensing detector acts to subtract the background or internal noise of the receiver.
MMW radiometers are typically characterized into two common types, total power radiometers and Dicke radiometers. Total power direct detection radiometers are the simplest type and require fewer RF components, but suffer from both short-term and long-term gain variation. Total power receivers require good thermal control of the components to minimize gain variation. A Dicke radiometer employs an RF switch coupled between an antenna and a radiometer receiver thereby allowing the receiver to alternate between the antenna and a known reference load termination. The receiver output is connected to a synchronous detector that produces an output voltage proportional to a difference between the antenna and the reference temperature. Null balance operation for the Dicke radiometer has been achieved by coupling in noise from a hot noise diode to the antenna port of the RF switch thereby enabling matching the temperature from standard reference loads.
As described in FIGS. 1 and 2 of U.S. Pat. No. 6,217,210B1, current methods for space measurement of thermal radiation or brightness temperatures rely on the use of external cold sky horns or beams switched to cold outer space in order to obtain a single cold calibration temperature. As described in FIG. 3 of U.S. Pat. No. 6,217,210B1, ground based radiometers most often use internal heated known temperature loads and hot noise source diodes to obtain a single hot calibration temperature.
Thus, there is needed a Dicke-type radiometer that provides improved flexibility and measurement accuracy with less need for thermal control of critical receiver components. Also lacking in the prior art is an automatic null balance radiometer utilizing a programmable cold/warm noise source that can match the range of antenna temperatures that are typically between 50 to 300xc2x0 K.
The present invention is an improved radiometer for detecting and measuring MMW signals, including electrical noise signals of the thermal noise level type and comprising a Dicke-type comparison radiometer with an adjustable cold/warm noise source in a feedback loop.
In accordance with the present invention, there is provided a method of radiometric temperature measurement comprising coupling a brightness temperature signal to an antenna/calibration switch. The antenna/calibration switch is actuated into a measurement mode to couple the brightness temperature signal to a radiometer receiver. An output of the radiometer receiver is coupled to an adjustable cold/warm noise source having an output representing calibration temperatures and coupled to the antenna/calibration switch. The cold/warm noise source is adjusted in accordance with the output of the radiometer receiver for a null comparison measurement.
Further in accordance with the present invention, there is provided apparatus for radiometric temperature measurement comprising a radiometer receiver having an input coupled to an antenna/calibration switch. The antenna/calibration switch enables selection between an antenna mode and a calibration mode. A driver coupled to the antenna/calibration switch controls the operation of the switch between the antenna mode and the calibration mode. In the antenna mode a brightness temperature is applied to the input of the radiometer receiver. An adjustable cold/warm noise source is coupled to the antenna/calibration switch, the noise source provides a calibration radiation temperature applied through the antenna/calibration switch in the calibration mode to the radiometer receiver. Coupled to the output of the radiometer receiver and the input of the adjustable cold/warm noise source is a feedback loop that provides a feedback signal to the noise source to adjust the calibration radiation temperature to match the antenna temperature in accordance with the output of the radiometer receiver.
According to another embodiment of the invention, there is provided a method for dual mode radiometric temperature measurement comprising coupling a brightness temperature signal to an antenna/calibration switch. The antenna/calibration switch is actuated into a measurement mode to couple the brightness temperature signal to a radiometer receiver having an output coupled to a mode selector switch. The mode selector switch is operated into a mode one position to couple the output of the radiometer receiver to an adjustable cold/warm noise source. The antenna/calibration switch is actuated into a calibration mode to decouple the brightness temperature signal from the radiometer receiver and to couple the adjustable cold/warm noise source to the radiometer receiver. A bias source is also coupled to the mode selector switch and actuation of the mode selector switch into a mode two position couples the bias source to the adjustable cold/warm noise source. The bias source is adjusted to vary the output of the adjustable cold/warm noise source applied through the antenna/calibration switch to the radiometer receiver.
In addition, according to the present invention, there is provided apparatus for dual mode radiometric temperature measurement comprising a radiometer receiver coupled to an antenna/calibration switch. The antenna/calibration switch operates between an antenna mode and a calibration mode, in the antenna mode a brightness temperature signal is applied to the radiometer receiver. A driver is coupled to the antenna/calibration switch for controlling the operation of the switch between the antenna mode and the calibration mode. An adjustable cold/warm noise source is also coupled to the antenna/calibration switch, the noise source having an output representing a calibration radiation temperature applied through the antenna/calibration switch in the calibration mode to the radiometer receiver. A mode selector switch for selecting between a null balance mode and a calibration mode is coupled in a feedback loop between the output of the radiometer receiver and the input of the adjustable cold/warm noise source. The mode selector switch operates between a null balance mode and a calibration mode. A bias signal source is coupled to the mode selector switch for applying a bias signal to the adjustable cold/warm noise source through the mode selector switch when in the calibration mode.
A technical advantage of the present invention is a comparison type radiometer having an adjustable cold/warm noise source in a feedback loop for automatic null balance operation. Automatic null balance minimizes or substantially eliminates the effects of receiver gain variation, and bandwidth change with temperature. This reduces the requirement for thermal temperature control of the radiometer to only the adjustable cold/warm noise source thereby reducing power consumption. Another technical advantage of the present invention is a dual mode closed loop and open loop Dicke-type radiometer that when operating in the open loop calibration mode utilizes selected calibration temperatures that cover the range of the antenna temperatures.